Threaded joints for tubes are popularly used to connect steel tubes used in petroleum installations of oil industry such as oil country tubular goods. In connecting steel tubes used for search and production of oil or gas, conventionally, a standard threaded joint stipulated in API (American Petroleum Institute) standard has been typically used.
However, recently, since deep wells for crude oil or natural gas have been in progress and horizontal wells and directional wells on behalf of vertical wells have been increasing, excavation and production environments are becoming severe. Further, wells developed in an appalling circumstance such as oceans and polar regions have been increasing. Hence, the performance threaded joints have to satisfy is diversified including compression resistance, bending resistance and sealability against external pressure (external pressure resistance). In view of the above, the use of a special threaded joint having high performance referred to as a “premium joint” is increased, and a demand for the improvement in performance of the premium joint is also steadily increased.
The premium joint is a coupling-type joint where an externally-threaded member (hereinafter referred to as “pin”) includes a tapered thread, a metal-to-metal seal portion (hereinafter referred to as a seal portion) and a torque shoulder (hereafter referred to a shoulder) and is formed on a pipe end portion, and an internally-threaded member (hereinafter referred to as “box”) including a tapered thread, a seal portion (to be more specific, a metal touch seal portion) and a shoulder (to be more specific, a torque shoulder) and connects the pin to each other are joined to each other. The tapered thread is important to firmly fix the tube joint. Bringing the box and pin into a metal contact at the seal portion makes this portion play a role in ensuring sealability. The shoulder forms a shoulder face that functions as an abutment during making up the joint.
FIGS. 3A to 3C are schematic explanatory views of a premium joint for oil country tubular goods, which are vertical cross-sectional views of a threaded joint for a cylindrical tube. The threaded joint includes a pin 3a and a box 1a into which the pin 3a is fitted. The pin 3a has, on the outer surface thereof, an externally-threaded member 7a and a nose (also referred to as a pin nose) 8a which is a portion formed adjacent to the externally-threaded member 7a on an end of the pin 3a and has no threads. The nose 8a has a seal portion 13a on the outer peripheral surface thereof, and a shoulder 14a on the end surface thereof. The box 1a into which the pin 3a is fitted is a portion having an internally-threaded member 5a, a seal portion and a shoulder 12a on the inner surface thereof, and these portions 5a, 11a, and 12a are portions threadedly engaged with or brought into contact with the externally-threaded member 7a, the seal portions 13a and the shoulder 14a of the pin 3a, respectively. In FIG. 3A, a chain line indicates a tube axis.
In the conventional example shown in FIG. 3C, the threaded joint is a so-called pin end seal type premium joint where the seal portions 11a and 13a are positioned at the end portion of the nose 8a. Apart from such a premium joint, there has been also known a radial seal type premium joint where the seal portions 11a and 13a are positioned at an intermediate portion of the nose 8a in a tube axis direction, and the pin 3a and the box 1a are brought into metal-to-metal contact in a tube radial direction.
To install a tube in a well, there may be a situation where the tube is installed while being rotated. In digging a directional well or a horizontal well, the tube is rotated in a state where the pipe is bent during digging. In this case, tensile-compressive forces are repeatedly applied to the tube. Along with the progress of deep wells, the above-mentioned tensile-compressive forces are also increased. Due to such repeated tensile-compressive forces, plastic deformation occurs in a threaded portion and a seal portion of a box which constitutes a threaded joint, and there may be a situation where sealability is lowered or cracks develop resulting in rupture in a worst case.
In view of the above, as a threaded joint structure exhibiting strong resistance against repeated tensile-compressive forces or repeated bending forces, there has been proposed a structure that alleviates stress concentration by arranging a spiral stress relaxation chase on threaded bottoms of one end or both ends of threadedly meshing end portions of either one or both of pin side and box side (see for example, Japanese Patent No. 3401859 (Japanese Patent Application Publication No. 7-98082)).
However, a stress or a strain acting on a threaded corner portion of a box side is not determined only based on the shape of threads and the above-mentioned stress relaxation chase, and the stress or the strain is influenced by the tolerance of a cross-sectional area of a weakest portion of the box with respect to a cross-sectional area of a tube body. Hence, there exists a drawback that the stress concentration at the threaded corner portion of box side cannot be sufficiently alleviated only based on the shape of threads or the arrangement of the stress relaxation chase.
It could therefore be helpful to provide a threaded joint structure where the stress concentration hardly occurs at the threaded corner portion of box side even when the threaded joint receives repeated tensile-compressive forces or repeated bending forces.